Like all good universities, a post on the faculty of the Unseen University is a post for life. That does not mean, however, that a young appointee will be in post for very long.

The Keeper of Doors is an ancient office in the Unseen University. The holder of the title has impressive robes, a seat at High Table and the gratitude of his colleagues, who might otherwise have found themselves in his place. New appointees get the rooms vacated by the previous incumbent, his half-written and usually in-decipherable notes, and books of 'Door Lore' written by a few really competent past Keepers. They enjoy their undoubted status at first, but sooner or later the urge to use one of Doors in their keep becomes overwhelming, and one day it is noticed that their reserved seat at High Table hasn't been used for a while.

The last incumbent managed to stay in office for a record five years, mainly due to a total lack of both curiosity and imagination. After five years he was found in an obscure corner of the library, hiding, he said, from an invisible crocodile. This made one wizard very happy, having won the traditional sweepstake.

The newly appointed Keeper found that his duties were slight. He must daily inspect the Doors, and once a year show them to interested members of the University. The first time this occurs - if he lasts in post long enough for it to happen - usually attracts a few wizards interested in judging the man, and perhaps to see whether their sweepstake ticket has any chance of coming up.

So it was that a crowd of wizards, just large enough to be difficult to count, accumulated outside the great door which barred the entrance to the corridor of Doors. The crowd included both the Archchancellor and the Librarian. Others not present were any of a nervous disposition or an aversion to risk.

The Keeper made his way to the great door, peered closely through the little barred window, threw back the great bars, and unlocked the great door with the great brass key which he wore round his neck on a black ribbon as his symbol of office. He peered through the little barred window again and then opened the door inwards.

"After you, gentlemen, please."

There was much mumbling and "no, no, after you …", but led by the Archchancellor, most of them filed into the broad corridor. The remainder suddenly remembered urgent appointments elsewhere.

"Is that," asked the Archchancellor, pointing to an intricately patterned rug on the floor someway down the corridor, "a magic carpet?"

"Yes Archchancellor, it is highly magical."

"Flies well does it?"

"No Archchancellor it doesn't fly; it is a non-flying magic carpet, and it doesn't move at all."

"Eh! What's the use of that?"

"According to the records, it covers a trapdoor in the floor leading to … to somewhere else. By not moving, the carpet prevents anything from coming up out of the trapdoor. I could roll up the carpet if you want to see it?"

"Perhaps another time. Eh, gentlemen?"

There was a general murmured agreement.

"Ok, gentlemen, then let us start with our smallest and safest door." The Keeper produced a tiny silver key, and kneeling down unlocked a door so small he could not have put his head through it. "If you care to look, gentlemen, you will see a garden. It is always there, always much the same, day or night, summer or winter."

The Librarian, far more nimble than the others, got down on all fours and peered in. "Ook?"

"I've sent in imps, Librarian, they look around and come out again. All they'll say is that it's a garden. Imps are not interested in gardens, you see."

"How about gnomes?" someone asked.

"I've tried," said the Keeper. "Gnomes like gardens all right; the trouble is they go in and don't come out again. I tied a length of string to one and let him in. When I pulled him back out he just sulked and would say nothing."

He locked the tiny door and moved down the corridor. Here the roof was high to accommodate a huge metal ring mounted on the wall. It was large enough that a horse and cart could be driven through it, if the solid rock of the corridor wall wasn't there. It was covered in symbols nobody could interpret, and few had ever seen before.

"This," the Keeper announced grandly, "is our largest Door, if that is what it is. Nothing much is known about it. According to the records it is called a 'stare gait'." He spelt it out. "I suspect that is a miss-spelling, and it is really a gate. Stair gate?"

Everyone looked blank, so he moved on quickly to a dark curtain. "Look closely, gentlemen, and you will see movement. The whole curtain appears to flow from left to right, though it never moves. It is called the 'River Curtain.' Sometimes, behind the curtain you will find a narrow side passage leading off; it isn't always there. I've see it just once."

"Ook?"

"No Librarian, I didn't. But once I've found a way of making sure that it stays open I will explore it."

The Archchancellor peered behind the curtain. "It seems to be there now, would anyone care to go in?"

"I don't recommend it Archchancellor," said the Keeper, his voice a little shrill, "One of my predecessors did go in and come back out. He found that at the end of the tunnel there is a trapdoor in its roof. Above that is what appeared at first to be a normal country house, but it was occupied by a giant toad, man-sized, with other animals to do its bidding."

There was a shudder all round, for wizards associate toads with witches and prefer to have nothing to do with them.

At the very end of the corridor was a bulky dark-wood wardrobe. The Keeper brought the party in front of it. "This wardrobe appears ordinary, but the wood at the back is magical – not sapient pear-wood but something a bit similar, some form of apple wood, though where it came from nobody knows."

"Yes, but what sort of Door is it?" came a call from the rear.

"If you hang clothes in the wardrobe, get into it, close the doors and slowly work your way to the back you may find yourself in a forest, with snow. It's a very magical place. Not everyone who has gone in has come out again, and those that have say it can seem like years finding the way back."

The Keeper talked about a few more Doors as the group made its way back down the corridor. The Archchancellor stopped him as they went past a pair of metal doors. "What about these, Keeper?"

"Oh, some Keeper long ago had that device installed. If you can open those sliding metal doors you'll find a tiny room with no furniture. At one time you could go in and close the doors, wait a few seconds, open them again and find yourself in the Library."

"Sounds rather a powerful magic."

"It does, doesn't it Archchancellor. However, it's simply an elevator."

The Archchancellor refused to be embarrassed: "Ho ho, jolly good joke, glad to help you with that one."

There was one last Door, a simple wooden door that opened with a handle, it had no lock. "Looks like a cupboard door doesn't it" said the Keeper brightly.

"And is it?" asked the Archchancellor.

"It appears to be," replied the Keeper. "Whenever I've opened it I've always found the same thing: a titchy room with hooks and rails for clothes. Like a big wardrobe."

"Any chance it's just that?" demanded the Archchancellor, thinking he was being set up again.

"It's what comes out of that Door, Archchancellor. Left to itself the Door may suddenly open and a monstrous thing come out."

"Oh, like that trapdoor under the carpet I suppose. What does it do when it 'comes out'?"

"Same thing every time: it scars silly whoever sees it, and then goes back in and closes the door."

"That's it?"

"Yes, put like that it doesn't sound too dangerous."

The Archchancellor, having decided that this new Keeper was far too cocky, blurted out: "Let us see this monster; invoke it for us."

"What now, Archchancellor?"

"Yes now, man, if you can do it!"

"I could try, Archchancellor, but there are too many of us here. This monster is cautious, and it won't come out unless conditions are right. Perhaps I could persuade some of you fellows to leave…"

Very soon the Keeper was left with the Archchancellor, Ponder Stibbons, and two other wizards he did not know. He closed and locked the great door at the end of the corridor with his great brass key: "We don't want whatever comes out escaping into Ankh-Morpork, we must face it alone, no matter what."

He looked at each remaining man, and quickly picked on Ponder: "Mr Stibbons, you will do nicely, please be so good as to hang your robe in the little room. Go on, it won't bite!"

"Good, leave the Door very slightly ajar. I'll dim the light; you lie on the floor, on the unmoving carpet if you like, and pretend you're asleep."

The bait lay down, not sure if he was being made a fool of or being placed in extreme jeopardy.

"Excellent, gentlemen. I suggest that we move back into the shadows, keep quiet and wait. It is essential, Mr Stibbons, that you don't actually fall asleep. The important thing is that you are relaxed and sleepy, but awake. If it helps, keep your eyes on the Door. And remember, we are all here to defend you, come what may."

Ponder Stibbons thought that there was fat chance of him relaxing, never mind falling asleep; but after a few minutes he did relax, possibly as a side-effect of close contact with the un-moving carpet. A few minutes later his eyes were beginning to close when he saw the door slowly swing open.

Behind the door there was a faint light which should not have been there, and then a strange, six-legged creature emerged. It was less than three feet high, and resolved into a large ball on two spindly legs carrying a four-legged stool. It sat down on the stool with its one enormous eye fixed on the sprawled form of Ponder, who was staring straight back at it.

"Uh Oh! You're not a child are you?" it asked rhetorically, getting off the stool and backing slowly towards the Door.

"He is a wizard, as are we all. Stand still and no harm will come to you" called the Keeper, stepping forwards, a glowing wand held high. But even as the light became bright, the creature was through the Door taking its stool with it. The Door slammed shut. The Archchancellor yanked it open: there was the tiny room with Ponder's robe hanging in it. The Keeper ran in and tried banging the back wall: it was solid. He looked up for a trapdoor, and down for a trapdoor: nothing. He tried closing and opening the door from either side: it was just a door.

Meanwhile the Archchancellor's laughter was booming in the corridor. "Did you see the colour of it? And that mouth! Door keeper, you've got your doors muddled: this is the one with the 'giant toad' behind it."

The Archchancellor would not let the matter drop; there was derisive laughter at the Keeper's expense every time he showed his face. His standing gone, the Keeper resigned his position and left the Unseen University. There was considerable dispute after that: which sweepstake ticket covered leaving as a laughingstock? The bickering went on for weeks.